The present application claims the priority benefit of the European Patent Application No. 01201550.9 filed Apr. 27, 2001, under 35 U.S.C. xc2xa7119, which is herein fully incorporated by reference.
1. Field of the Invention
The invention relates generally to image forming devices that are used in image reproduction systems, e.g. copiers or printers, in which a toner image is formed on a surface of an image forming element. Specifically, the invention relates to a so-called direct imaging process in which toner particles from a supply of toner in an image forming zone, are directly deposited on an insulating surface as a result of electrical energization of a printing electrode.
2. Discussion of the Related Art
Such direct imaging process are well known and are described, e.g., in U.S. Pat. No. 3,909,258, EP 191521, EP 295532 and EP 304983.
The image forming element is typically formed by a cylindrical drum or an endless belt which moves past an image forming station where the toner powder is applied to the insulating surface of the drum or belt under the control of electronic drivers and in accordance with the image information to be printed. The drivers control electrodes which generate an electric field for attracting the toner particles to the surface of the image forming element. A detailed description of the mechanism of toner deposition in a direct imaging process is provided in the above mentioned EP 191521.
The toner image that has been formed on the surface of the moving image forming element is then carried on to a transfer station where the toner image is transferred onto an intermediate image carrier or directly onto a recording sheet.
A malfunction of one or more of the drivers controlling the transfer of toner onto the surface of the image forming element will lead to a defect in the printed image. EP-A-0 991 259 discloses an image forming device having self-diagnosis means for detecting such malfunctions of the drivers by measuring an output characteristic of each of the drivers. The result of this self-diagnosis may be used for generating a signal advising the user that maintenance or repair is necessary. This signal may also identify the driver or drivers that are not functioning properly, so that the service personnel may readily take the necessary steps for exchanging or repairing the defective component. In addition, the result of the self-diagnosis may be used to activate correction means for automatically eliminating the malfunction or at least eliminating the visible effect thereof on the prints produced by the image forming device. For example, the malfunction may be eliminated by automatically activating a spare driver which will then take-over for the defective driver. As an alternative, the visible effect of a driver malfunction may be eliminated by automatically activating an image processing routine for modifying the image information to be printed such that the visible effect of the malfunction will be concealed as far as possible. However, the self-diagnosis means proposed in EP-A-0991259 can only detect a malfunction by reference to the output signals of the drivers, and they cannot verify the amount of toner that is actually deposited on the image forming element in response to the driver output signal.
It is an object of the invention to provide a method for directly detecting the amount of toner deposited on the surface of an image forming element, thereby to provide more powerful tools for self-diagnosis and/or self-correction in an image forming device and process.
It is another object of the invention to provide a method of detecting the amount of toner deposited on an image forming element, which overcomes the problems and disadvantages associated with the related art.
According to the invention, these objects are achieved by a method of detecting an amount of toner deposited on a surface area of an image forming element, the method including the step of measuring a change in the impedance/capacitance of an electrode extending over the surface area of the image forming element.
The invention is based on the effect that the amount of toner present on a given surface area of the image forming element will cause a measurable change in the impedance/capacitance of an electrode that extends over this surface area. Thus, the presence of toner on this surface area may be detected by reference to the measured impedance/capacitance, and the amount of toner may even be determined quantatively on the basis of a unique relation between the impedance/capacitance of the electrode and the deposited amount of toner. This unique relation may be determined experimentally in advance.
According to the invention, the surface area on which the amount of toner is detected will be defined by the configuration of the electrode and may incorporate the entire surface of the image forming element or only, e.g., a small portion thereof having, for example, the size of one or more pixels or a complete line or row of pixels. If, depending on the type of image forming process, the surface layer of the image forming element happens to be electrically conductive, then the electrode may be formed by the surface layer of the image forming element itself. On the other hand, if the image forming element has an electrically insulating surface layer, then the electrode may be embedded in the image forming element underneath this surface layer. Optionally, the electrode may also be arranged outside of the image forming element so as to face the surface thereof. In order to obtain a good signal-to-noise ratio, it is only required that the electrode is sufficiently close to the surface area of the image forming element, so that the dielectric properties of the toner deposited on this surface will influence the capacitance of the electrode. If, in case of an electrostatic image forming process for example, the image forming element has electrodes for generating an electric field that will attract the toner particles, then this electrode may conveniently be used for capacitive toner detection.
Measuring devices for measuring the impedance/capacitance of an electrode with high accuracy are, as such, well known and are used for example for capacitively measuring the thickness of plastic films and the like.
In one embodiment of the invention, the method of measuring the capacitance of the electrode comprises the steps of connecting the electrode to a voltage source and charging the electrode to a first predetermined potential, disconnecting the electrode from the voltage source and connecting it to a second predetermined potential, e.g. to ground, through an electronic integrator, and integrating the current flowing through the integrator while the electrode is discharged to the second predetermined potential. The result of the integration represents the change in the electric charge of the electrode, and by dividing this change of charge through the difference between the first and second predetermined potentials, one obtains directly the capacitance of the electrode. The integrator may for example be formed by an operational amplifier. Since, in this case, the discharge resistance for the electrode can be made very low, the capacitance can be measured with high accuracy even when the electrical insulation of the electrode from its environment is poor.
According to the present invention, another possible method of measuring the change in the capacitance of the electrode due to the toner being deposited on the surface of the image forming element may include keeping the potential of the electrode constant and measuring the amount of charge flowing to or from the electrode while the toner is being deposited on the image forming element. The measured charge divided by the constant potential of the electrode will then give the change in capacitance that has been caused by the toner.
In general, it is possible with existing technology, e.g., with Charge Coupled Devices (CCDs), to measure small electric charges even as small as a single electron charge with extremely high accuracy, e.g. in the order of a fC (10xe2x88x9215C) or less, making it possible to detect extremely small amounts of toner and even to detect a single toner particle.
If the electrode and hence the surface area defined thereby has only very small dimensions at least in one direction, e.g. if it consists of a single pixel or a single row of pixels, then the impedance/capacitance may be influenced not only by the toner deposited on this surface area itself but also by the toner deposited on adjacent surface areas. These xe2x80x9cedge effectsxe2x80x9d may however be taken into account when determining the relation between the impedance/capacitance and the amount of toner on the surface area.
The invention further relates to image forming methods and devices utilizing the above method of detecting the amount of toner.
When the toner detection method of the present invention is used for self-diagnosis or preventive maintenance purposes, it is possible to monitor not only the functions of the drivers controlling the direct imaging process but also the functions of other components and other parameters that have an impact on this process, e.g. the function of a toner supply system, changes in the surface properties of the image forming element, changes in the properties and composition (e.g. particles size distribution) of the toner and the like. When this method is combined with the means for monitoring the output signals of the drivers as described in EP-A-0 991 259, it is possible to provide a detailed diagnosis result which permits to facilitate and speed-up maintenance and repair operations to a considerable extent.
Moreover, the toner detection method of the present invention may be used for enhancing and adding self-correction facilities in an image forming device. If it is detected for example in an electrostatic direct imaging device that the amount of toner deposited on the image forming element is, for any reason, smaller than desired, then this effect may be compensated by modifying the output signal of the driver, e.g. by increasing the voltage applied to the imaging electrode and/or the counter electrode, so that the deposited amount of toner is increased. In this way, it is possible to control the optical density of the printed image with unprecedented accuracy. It will be understood that this is particularly useful in colour printing or copying operations in which the hue of the colour image depends critically on the optical densities of the various colour components.
The toner detection method of the present invention may also be used for improving other correction measures that are implemented already in existing image forming devices. For example, if a driver associated with a given pixel position on the image forming element outputs a pulse signal with a pulse duty ratio of 50% while the image forming element moves past the image forming station, this would result in a one pixel-wide broken line being drawn on the image forming element, and the average optical density of this line should, theoretically, be 50%. In practice, however, the average optical density of the line will not be 50%, but will be slightly smaller or larger, depending on the properties and conditions of the image forming process. A known method for compensating this type of error includes lengthening or shortening the xe2x80x9conxe2x80x9d periods of the pulsed signal output from the driver, e.g. by advancing or retarding the trailing edge of each pulse by a certain delay time. Now, the present invention offers the possibility to adapt this delay time dynamically in accordance with the actual optical density that is determined by reference to the measured amount of toner.
Such self-correcting or self-adjusting features of the present invention may be implemented in an image reproduction system employing the image forming device by causing the system to perform a self-test either upon a user instruction or in regular intervals. Alternatively, a self-test operation may be performed automatically each time an image has been printed. Since the measurement of the amount of toner can be performed within extremely short time, it is even possible to perform a self-adjusting operation continuously and essentially in real-time while an image is being printed. Eventually, this leads to an image forming method in which the drivers are feedback controlled on the basis of the measured amount of toner, so that the optical density of the image being formed is controlled to a target value with high reliability and accuracy.
This concept may be developed further to provide a halftone image forming process. Most commonly used image forming devices are only capable of printing either black or white pixels. Halftones are generated by dividing the pixel into a regular or irregular pattern of sub-pixels, on the cost of image resolution, with the grey value of the pixel as a whole being determined by the ratio between black and white sub-pixels. Since it is possible with the toner detection method according to the invention to measure the amount of toner applied to an individual pixel quantitatively, the amount of toner for a given pixel can be controlled so as to correspond to a desired grey value. Thus, since it is no longer necessary to divide the pixel into sub-pixels, halftone images can be printed with extremely high spatial resolution.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.